ReviewGreen concrete: Prospects and challenges
Introduction
Globally, management of solid wastes poses a herculean challenge to developed and developing countries owing to industrial growth, construction booms, rapid urbanization, and consumeric lifestyle [1]. The demand for green concrete in construction industry is spurred by increased regulations to reduce carbon footprint, limit greenhouse gas emission and limited landfill spaces. In addition, the construction industry is embracing green construction owing to project requirements for LEED (Leadership in Energy and Environmental Design) certifications.
The present high demand for natural resources to meet infrastructural demands has created immense opportunities for the use of waste materials to green infrastructure construction [2], [3], [4], [5]. These waste materials play the roles of either supplementary cementitious materials (SCM) or alternative aggregates (AA) in green concrete and can be categorized as agricultural, industrial and municipal wastes as shown in Fig. 1.
Though coined in Denmark in 1999, green concrete has been in practical existence for several decades and centuries. Jin and Chen [6] defined green concrete as concrete produced by utilizing alternative or recycled waste materials in order to reduce energy consumption, environmental impact and natural resource consumption. Green concrete is a concept of embracing and integrating environmental considerations in concrete with respect to raw material sourcing, mix design, structural design, construction and maintenance of concrete structures [7].
The inherent drawbacks of traditional concrete include unsustainable consumption of natural raw materials, low, early-age compressive strength, environmental contamination [8], [9], [10].
On the other hand, green concrete exhibit numerous advantages such as improvement in concrete properties, low carbon footprint, conservation of natural resources, to mention a few [11].
The major barriers for green concrete utilization in construction are systemic lock-in, lower qualities of locally available materials, increase in construction costs, and technical barriers [6], [12].
In order to produce sustainable green concrete, technological advances that are energy efficient, utilize low-carbon production methods and novel cement formulations are required alongside technical guidelines for their production and usage [13].
Section snippets
Common waste materials used as SCM in green concrete
The waste materials utilized in green concrete can be grouped into three categories namely agricultural, industrial and municipal wastes as depicted in Fig. 1. In order to utilize their pozzolanic properties in green concrete, the waste materials are often activated through physical or chemical means or their combination [14], [15].
Activation techniques
Activation is necessary to prevent slow and low, early-age strength development and accelerate the pozzolanic reactivity of SCMs in green concrete. Activation helps to achieve higher early and later strength amongst other benefits [89]. Types of activation techniques available in literature include mechanical activation, chemical activation, curing/temperature activation, water-controlled activation and SCM-controlled activation.
Mechanical activation involves grinding of SCM to smaller fine
Production of green concrete
Production methods of green concrete differ depending on the constituent materials to be utilized and the intended application. In order to produce sustainable, green concrete with sufficient workability, Müller et al. [117] suggested four basic steps namely:
- I.
Determining experimentally the relevant properties of the selected concrete constituents
- II.
Determine the water/cement ratio based on desired cement content and strength requirements
- III.
Optimize the grain size distribution of granular constituent
- IV.
Slump and water requirement
Slump test indicates the behavior of compacted concrete cone under the action of gravitational force, which can also be seen as a measure of the consistency or wetness of the concrete mix [129].
In order to produce HVFAC, Bentz et al. [130] recommended optimum mixture proportioning and careful selection, evaluation and combination of HRWRA (high-range water-reducing admixtures) alongside increasing aggregate volume fraction. Alaka and Oyedele [131] obtained good workable HVFAC at low
SCM chemical composition
Comparison of the chemical composition of the five (5) different SCMs and OPC revealed that, on the average, SF has the highest (silica content), followed by RHA. Also, it was observed that fly ash recorded the highest (alumina) content followed by GGBFS. In terms of CaO (calcium oxide) content, OPC recorded the highest value followed by GGBFS as depicted in Fig. 20.
Water/binder (w/b) ratio
Hu et al. [14] observed that higher water/cement ratio leads to lower ratio, large pores, higher porosity
Binary, ternary and quarternary SCM mixtures
The concept of binary, ternary and quaternary SCM is to obtain blended SCM with properties that are superior than the individual SCM constituents. Utilization of such blended cements overcomes the drawbacks associated with any of the individual constituent and maximizes their individual strengths or advantages. While Rakhimova and Rakhimov [66] recommended a component-wise approach in the development and application of sustainable cement and green concrete, Wang and Chen [273] presented a
Nomenclature and applications of green concrete utilized in concrete structures
Existing literatures on green concrete revealed the existence of different nomenclatures for green concrete depending on the SCM utilized, properties of the green concrete such as compressive strength, performance levels, compactability and density as depicted in Fig. 24. They include HVFAC (high-volume fly ash concrete), UHPC (ultra-high performance concrete), HPC (high performance concrete), ultra-high strength concrete (UHSC), HSC (high strength concrete), SCC (self-consolidating concrete),
Analytical and numerical modelling of green concrete
Proper experimental investigation is essential for reliable and accurate analytical and numerical modelling of green concrete and its properties. The three methods, experimental, analytical and numerical, should be viewed as complementary means to comprehensively understand, analyze and predict the behavior/response of both green concrete and ordinary concrete within the confines of available limited literatures. It must be borne in mind that each of the three methods presents peculiar
Potential benefits of green concrete in early project completion and cost savings
According to Hong Kong Design Code, the lowest grade of concrete for use in reinforced concrete is C20 (20 MPa). For construction of multi-storey, minimum concrete grade of 25 MPa is often required and utilized [393]. For high-rise buildings, HSC are often utilized with concrete strength ranging from 55.1 to 131 MPa [393], [394]. During concrete construction projects, allowance of 28 curing days is given for concrete works including columns, slabs and beams to develop sufficient strength in
Future trends in production and application of green concrete
Green concrete can be used in blocks, floor screeding underlays and façade panels [417]. Green concrete is foreseen to be applied more in pre-fabricated construction technology because it is more environmentally friendly than traditional cast-in-situ concrete technology [418].
GGBFS-based green concrete is used in mass concreting to limit and control temperature rise because of its lower heat generation compared to OPC [419]. UHSC is currently limited to offshore and marine structures,
Current challenges and obstacles
Some obstacles faced in green concrete applications in the construction include difficulties in compliance with regulatory standards such as minimum clinker concrete levels and chemical composition of cements, lack of or insufficient durability data of spanning up to 20 years or more, differentiation of green concrete for different applications, more research & development to promote better understanding of the chemistry of green concrete [386]. This necessitates the revision of various
Conclusion
Green concrete comes in various forms such as high-strength concrete, ultra-high performance concrete, ultra-high strength concrete, self-consolidated concrete, high-performance concrete, lightweight concrete, high-volume fly ash concrete and geopolymer concrete. The approaches that would be adopted to encourage green concrete in construction would be different in each country because of differences in development priorities, capacity and skill level of local construction industry.
Utilization
Acknowledgments
The authors gratefully acknowledge UGC-Postgraduate Studentship Hong Kong Government Award/funding given to Sojobi A.O. towards his PhD programme in the Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China. Sojobi A.O. appreciates the guidance and support of colleagues towards the writing of this manuscript. The authors appreciate the constructive feedback from the reviewers which led to significant improvement of this manuscript.
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